Saturday, January 29, 2011

Fusion Energy A Step Close To Reality: Plasma Stability Controlled

If you haven't heard about ITER, chances are you will soon. The scale and scope of the ITER project rank it among the most ambitious science endeavors of our time. With the Organization in place and site work completed, scientists are now poised to begin construction on the buildings that will house the ITER fusion experiments. Scroll down for some interesting facts about the project.

The ITER Tokamak will weigh 23 000 tons. The metal contained in the Eiffel Tower can't compare - it only weighs 7 300 tons. The ITER Tokamak will be as heavy as three Eiffel Towers.

Incorporation of control coils in the plasma vessel of the ASDEX Upgrade fusion device
Photo: IPP; Volker Rohde

Fusion is the process at the core of our Sun. What we see as light and feel as warmth is the result of a fusion reaction: Hydrogen nuclei collide, fuse into heavier Helium atoms and release tremendous amounts of energy in the process.

 (Click to view larger version...)
Credit: ITER

In the stars of our universe, gravitational forces have created the necessary conditions for fusion. Over billions of years, gravity gathered the Hydrogen clouds of the early Universe into massive stellar bodies. In the extreme density and temperature of their cores, fusion occurs.
Fusion is the energy source of the Universe, occuring in the core of the Sun and stars. (Click to view larger version...)
Credit: ITER

Atoms never rest: the hotter they are, the faster they move. In the core of our Sun, temperatures reach 15 000 000° Celsius. Hydrogen atoms are in a constant state of agitation, colliding at very great speeds. The natural electrostatic repulsion that exists between the positive charges of their nuclei is overcome, and the atoms fuse. The fusion of two light Hydrogen atoms (H-H) produces a heavier element, Helium.

The mass of the resulting Helium atom is not the exact sum of the two initial atoms, however: some mass has been lost and great amounts of energy have been gained. This is what Einstein's formula E=mc² describes: the tiny bit of lost mass (m), multiplied by the square of the speed of light (c²), results in a very large figure (E) which is the amount of energy created by a fusion reaction.

Every second, our Sun turns 600 million tons of Hydrogen into Helium, releasing an enormous amount of energy. But without the benefit of gravitational forces at work in our Universe, achieving fusion on Earth has required a different approach.

20th century fusion science has identified the most efficient fusion reaction to reproduce in the laboratory setting: the reaction between two Hydrogen (H) isotopes Deuterium (D) and Tritium (T). The D-T fusion reaction produces the highest energy gain at the 'lowest' temperatures. It requires nonetheless temperatures of 150 000 000° Celsius to take place - ten times higher that the H-H reaction occurring at the Sun's core.

At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma - a hot, electrically charged gas. In a star as in a fusion device, plasmas provide the environment in which light elements can fuse and yield energy.

In ITER, the fusion reaction will be achieved in a tokamak device that uses magnetic fields to contain and control the hot plasma. The fusion between Deuterium and Tritium (D-T) will produce one Helium nuclei, one neutron and energy.
Three, two, one ... We have plasma! Inside the European JET Tokamak, both before and during operation.
Photo: EFDA, JET.

Compensation of edge instabilities in ASDEX Upgrade successful pointing the way for ITER

The Helium nucleus carries an electric charge which will respond to the magnetic fields of the tokamak, and remain confined within the plasma. However some 80% of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the tokamak, transferring their energy to the walls as heat.

In ITER, this heat will be dispersed through cooling towers. In the subsequent fusion plant prototype DEMO and in future industrial fusion installations, the heat will be used to produce steam and - by way of turbines and alternators - electricity.

Fusion Reactor

Credit: ITER

After barely a year of modification work the first experiments conducted have already proved successful. Eight magnetic control coils on the wall of the plasma vessel of the ASDEX Upgrade fusion device have now succeeded in reducing perturbing instabilities of the plasma, so-called ELMs, to the level required. If these outbursts of the edge plasma become too severe, they can cause major damage to the plasma vessel in devices of the ITER class. The results now achieved go a long way towards solving this important problem for ITER.

The research objective of Max Planck Institute for Plasma Physics (IPP) at Garching is to develop a power plant that, like the sun, derives energy from fusion of atomic nuclei. Whether this is feasible is to be demonstrated with a fusion power of 500 megawatts by the ITER (Latin for “the way”) experimental fusion reactor, now being built at Cadarache, France, as an international cooperation. This requires that the fuel, an ionized low-density hydrogen gas – a plasma – be confined in a magnetic field cage without touching the wall of the plasma vessel and heated to ignition temperatures of over 100 million degrees.

The complex interaction between the charged plasma particles and the confining magnetic field can cause all kinds of perturbations of the plasma confinement. Edge Localized Modes (ELMS) are very much under discussion at present in relation to ITER. These cause the edge plasma to briefly lose its confinement and periodically hurl bundled plasma particles and energies outwards to the vessel wall. Up to one-tenth of the total energy content is thus ejected. Whereas the present generation of medium-sized fusion devices can easily cope with this, it might cause overloading in large-scale devices such as ITER of, in particular, the divertor – specially equipped collector plates at the bottom of the vessel, to which the plasma edge layer is magnetically diverted. This would make continuous operation inconceivable.

This ELM instability is, however, not altogether unwelcome, because it expels undesirable impurities from the plasma. Instead of the usual hefty impacts the aim is therefore to achieve weaker but more frequent ELMs. The 300-million-euro decision, originally scheduled for last year, on how to achieve this tailor-made solution for ITER was postponed by the ITER team, pending incorporation of the control coils in ASDEX Upgrade. This was because other fusion devices using similar coils – DIII-D at San Diego being the first – came up with conflicting results.

The experiments on ASDEX Upgrade now pave the way to clarification: Shortly after the power in the new control coils is switched on, the ELM impacts decline to a harmless level. But they occur often enough to prevent the accumulation of impurities in the plasma. The good confinement of the main plasma is also maintained. The ELMs do not regain their original intensity till the coil field is switched of. This experimental result goes a long way to answering the question how the energy produced in the ITER plasma can be properly extracted.

But the goal has not quite been attained: This is because the plasma edge of ITER cannot be completely simulated in smaller devices such as ASDEX Upgrade. It is therefore all the more important to understand exactly the processes underlying the suppression of ELMs; this calls for sophisticated measuring facilities for observation and a powerful theory group for clarification. The physical theory hitherto acquired at IPP does fit the present results, but has yet to be checked and expanded. Till the decision on ITER scheduled for 2012 there is time for solving the problem for the test reactor – and for a future power plant.

The possibilities afforded by control coils on ASDEX Upgrade are then still far from being exhausted: Another eight coils as of 2012 are to make lots of new investigations possible.

Max-Planck-Institut fuer Plasmaphysik

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